Varnish VLC configuration file
As we mentioned earlier, proxy servers follow specific rules when they handle requests. Varnish uses its own language for internal rules, called Varnish Configuration Language (VCL). In practice, VCL gives you full control over your proxy server’s configuration, so you’ll need to become acquainted with it first.

This Varnish Advanced Caching extension will help you generate this VCL configuration file, based on some configurable settings. There are a few important settings that will be used to generate the VCL file, like ipaddress and port uses by Varnish, different timeout values but also which ip addresses are allowed to PURGE/BAN the cache. 

Not all pages can be cached, like account, cart and checkout pages, these pages will be excluded from caching. You can also make your own list of urls which will be excluded from caching. 

Other settings like do you want separate cache for guests and customers (recommended), separate cache for customer groups (for instance when you have special customer group prices) or separate cache for desktop/tablet and smartphones, all those settings you can managed within Opencart itself by enabling/disabling those features. Thought special cache headers and cookies Varnish will know how to separate and store the page cache.

When you are able to use Ajax for loading customer specific data like the login/logout/account menu links, shopping cart and wish list data, it is possible to only cache one instance of a page and use it for both guests and customers. This Varnish Advanced Caching extension has already a build in Ajax loading for cart and wish list data, so pages with those specific data can be cached also.

When will the Varnish cache be invalidated/purged
Invalidating/purging the cache is very important to make sure you always serve up to date content to visitors. It is always possible to manually purge all cache or purge cache of a specific page like the home page or product pages. The Varnish cache will be automaticly purged when you as an admin make changes to products, categories and/or information pages.

The Varnish Caching extension has a built in advanced invalidation mechanism for purging and recache pages. Through Opencart events and a cron task every change in a product, category, information or manufacturer item will be evaluated and will trigger the right purge request to Varnish.

How to check that Varnish cache is working
Varnish caching system sets specific http headers if the called page is available or not in cache. To check if website is using Varnish cache, it is therefore necessary to check for the presence of specific Varnish headers.

Open your browser developer:
Firefox: Tools->Web development->Activate tools
Chrome: View->Development options->Developer tools

Now, on developer tools:

• Select “Network” tab
• Select “HTML” sub tab in Firefox or “Doc” in Chrome
• Refresh your page
• Click on the domain name row identified by 200 status code

The response headers will open in the right column, last header will be the one related to Varnish:

X-Cache: HIT = the resource/page called, is available in cache and has been returned by Varnish. It means that the cache system is working properly.

X-Cache: MISS = the resource/page called, is NOT available in cache and was generated by Opencart. A MISS is always returned when this was the first resource/page call (second call will be a HIT), or the cache system has been instructed by the headers or by custom rules, not to return the resource/page from cache.